An important operating characteristic of an integrated circuit is the temperature of the integrated circuit's die when the integrated circuit is fully powered and in a normal operating mode of operation. Such a temperature measurement is often desired at a final package test when the integrated circuit is in the same form as will be used by an end user. To satisfy this need, others have used an on-chip resistor connected to two dedicated external pins of the integrated circuit. The external pins have no other function than to implement the temperature measurement function. An example of an integrated circuit having two dedicated pins for temperature measurement of the integrated circuit die is the MC88110 reduced instruction set microprocessor.
The temperature of the integrated circuit having an on-chip resistor is measured by using a current source reference and forcing a known current thru the known resistance of the on-chip resistor. A resulting voltage across the two dedicated integrated circuit pins is then measured. The resulting voltage is proportional to the temperature of the operating integrated circuit. In other words, as the temperature of the operating integrated circuit die varies, the resistance varies proportionately which results in a proportional change in voltage measured across the two dedicated integrated circuit pins. The two dedicated integrated circuit pins are required in order to isolate internally generated noise and other circuit parameters from the two integrated circuit pins. Any voltage, current, resistance or impedance which is coupled to the two integrated circuit pins from internal to the integrated circuit will modify the voltage potential between the two integrated circuit pins and result in an incorrect temperature measurement. However, the price of two dedicated integrated circuit pins to implement a measurement function is costly, even in the most advanced very large scale integrated circuits.
Another type of integrated circuit which uses a plurality of resistors connected between the integrated circuit's external pins is a resistor network integrated circuit. However, this type of integrated circuit functions only to provide a user with a plurality of possible resistance values depending upon which pins of the integrated circuit are connected. No other operating circuitry is typically implemented in such circuits.
Yet another known type of integrated circuit which provides a signal from which an operating die temperature may be derived utilizes a significant amount of dedicated internal circuitry, such as an analog-to-digital (A/D) circuit and a storage register, to provide the signal at a single dedicated output pin of the integrated circuit. The additional internal circuitry which is required to implement this temperature measurement function is often prohibitive in many large scale integration designs.